A widely used and successful commercial process for synthesizing acetic acid involves the catalyzed carbonylation of methanol with carbon monoxide. Catalysts used in this reaction typically contain rhodium and/or iridium as well as a halogen promoter, for example methyl iodide. The carbonylation reaction may be conducted by continuously bubbling carbon monoxide through a liquid reaction medium in which the catalyst is dissolved. In addition to the catalyst, the reaction medium also may comprise methyl acetate, water, methyl iodide and the catalyst. Conventional commercial processes for carbonylation of methanol include those described in U.S. Pat. Nos. 3,769,329, 5,001,259, 5,026,908, and 5,144,068, the disclosures of which are hereby incorporated by reference. Also, U.S. Pat. No. 6,617,471, the disclosure of which is hereby incorporated by reference, discloses a vapor-phase carbonylation method for producing esters and carboxylic acids from reactants comprising lower alkyl alcohols, lower alkyl alcohol generating compounds, and mixtures thereof. Another conventional methanol carbonylation process includes the Cativa™ process, which is discussed in Jones, J. H. (2002), “The Cativa™ Process for the Manufacture of Acetic Acid,” Platinum Metals Review, 44 (3): 94-105, the disclosure of which is hereby incorporated by reference.
In the methanol carbonylation reaction, carbon monoxide, by-product gases, and feed impurities, e.g., hydrogen, nitrogen, argon, methane, and carbon dioxide, that are initially present in the carbon monoxide feed may build up in the reactor as the crude acetic acid product is formed. To improve overall reaction and catalyst efficiency and to prevent the build-up of these gases, a purge stream, e.g., an off-gas stream, is typically vented from the reactor. The vented purge stream may comprise carbon monoxide, inerts, volatile halogen promoters, acetic acid, water, unreacted methanol, methyl acetate and/or feed impurities. Typically, the purge stream as a whole is withdrawn directly from the reactor, e.g., the purge stream components are not withdrawn individually. Because the carbon monoxide in the purge stream is vented from the process, the same is not converted to acetic acid. As such, this loss of carbon monoxide contributes to a reduction in a carbon monoxide efficiency of the system. In addition to the carbon monoxide lost via the purge stream, residual carbon monoxide that remains in the crude acetic acid product after the primary reaction often is separated and purged, which further contributes to reductions in carbon monoxide efficiency. Accordingly, to improve overall carbon monoxide efficiency, conventional methanol carbonylation systems have attempted to reduce the size of the vented purge stream and the amount of carbon monoxide therein.
Even though the sizes of conventional purge streams are often minimized, the purge stream still may be processed to recover and/or utilize the components thereof. For example, the purge stream, once withdrawn, may be processed to recover volatile halogen promoters, acetic acid, water, unreacted methanol, and/or methyl acetate, which may in turn be recycled to the reactor. As another example, the small amount of carbon monoxide in the vent stream may be used to enhance catalyst stability, e.g., to reduce catalyst precipitation, in the units of the system.
As another example, U.S. Pat. No. 5,917,089 discloses that a purge stream, e.g., “off-gas” from the reactor may be fed directly to a second reactor, along with fresh methanol, to produce additional carbonylation product, i.e. acetic acid. The off-gas, as known in the art, is not a derivative stream. Although the carbon monoxide in the purge stream may be converted and efficiency may be slightly improved in this case, the amount of carbon monoxide available for catalyst stabilization is lessened.
Also, as noted above, conventional carbon monoxide feed impurities are also vented via the off-gas. Because the off-gas stream is vented from the reactor as a whole, conventional processes keep the amount of impurities in the carbon monoxide feed stream as low as possible, e.g., to minimize the loss of carbon monoxide that would accompany the unwanted impurities that are purged. Accordingly, conventional carbonylation processes often employ high purity carbon monoxide feed streams, which contain very low amounts of impurities. As a result of the very low amount of impurities, only smaller amounts of the off gas need to be vented from the reactor.
Generally speaking, the crude acetic acid product from the reactor is then processed via a purification train to remove impurities and provide a high quality acetic acid product. The purification train may include several vents, which purge non-condensable gases. The gases that are purged via the purification train vents may be processed in a recovery unit to recover light boiling point components, such as the halogen promoter, as described in US Publication No. 2008/0293966, the entire content and disclosure of which is hereby incorporated by reference. In most cases, at least a portion of the non-condensable gases, e.g., carbon monoxide, whether or not they pass through the recovery unit, are typically purged or flared. As noted above, this loss of residual carbon monoxide contributes to additional reductions in carbon monoxide efficiency.
Accordingly, in view of these references, the need exists for a methanol carbonylation process that provides 1) an increased amount carbon monoxide available for use as a catalyst stabilizer; and 2) an ability to use a lower purity carbon monoxide feed, while maintaining a high overall carbon monoxide efficiency.